With the improvement of photolithographic technology and the rapid development of semiconductor industry, the performance of a photolithographic apparatus can be assessed on the four fundamental characteristics, i.e., critical dimension (CD) uniformity, focal-depth or focus accuracy, overlay accuracy, and throughput. In order to improve the critical dimension uniformity, a wafer or a mask stage needs to improve its horizontal positioning precision. In order to improve the focal-depth accuracy, the wafer or mask stage needs to improve its vertical positioning precision. In order to improve the overlay accuracy, the wafer or mask stage needs to have an improved internal model which can enhance its dynamic positioning characteristics. In addition, considering the photolithographic apparatus is generally required to increase its throughput, the stage should also possess the capabilities of high-speed movement and rapid start and stop. The high-speed, high-acceleration and high-precision-positioning requirements of the photolithographic apparatus are conflicting. To increase the scanning speed necessitates more powerful motor capable of not only large-stroke and high-speed movement but also movement in multiple degrees of freedom for enabling lithographic exposure and alignment.
Photolithographic apparatuses can be generally categorized into two types. One is steppers, in which, a mask pattern is entirely exposed onto one of the target portions of a wafer at one exposure, and the wafer is then moved relative to the mask to locate the next target portion right under the mask pattern and a projection objective, where the mask pattern is exposed on the next target portion of the wafer. This process is repeated until the mask pattern is formed on each target portion of the wafer. The other is scanners, in which, the mask pattern is not formed at one exposure. Instead, the mask pattern is formed by the scanning and movement of a projection field. During the imaging of the mask pattern, the mask and the wafer are simultaneously moved relative to the projection beam and projection system.
Each of the above described photolithographic apparatuses employs a means to hold the mask/wafer, and it is the precise relative movement of the means for holding the mask/wafer that ensures the requirements for the photolithographic process to he satisfied. The means for holding the mask is called a mask stage, and the means for holding the wafer is called a wafer stage.
Turning now to FIG. 1, which shows a linear motor that uses conventional NS magnet arrays and a single-layered coil array.
U.S. Pat. Pub. No. 20040246458A1 discloses a linear motor for a wafer or mask stage of a photolithographic apparatus. The linear motor has a high driving force, high efficiency and low normal force, and includes a first magnet plate, a second magnet plate arranged in parallel and opposite to the first magnet plate, and an armature, which includes three open coil units, interposed between the first and second magnet plates. The first and second magnet plates and the coil units are relatively movable. The use of two opposed magnet plates and an open coil unit which does not include an iron core enables volume reduction of the motor without additional heat generation for an increase in the force output. Since this design leads to additional effective magnet material and hence higher magnetic forces, efficiency of the motor can be increased with the number of used bearings and the moving mass being both reduced, thus rendering the motor particularly useful in vacuum environments. However, as the reference has not considered the ripple force that may be caused by tilt or other factors, this motor is still incapable of satisfactory precision positioning of the stage. Moreover, the used laterally aligned cuboid-shaped magnets may cause magnetic flux leakage at the magnetic yoke and thus precision control difficulties and insufficient control forces. Therefore, there is a need for a novel positioning motor of higher-precision and a linear motor which is more powerful.
Further, with the improvement of photolithographic technology and the rapid development of semiconductor industry, the performance of a photolithographic apparatus can be assessed on the four fundamental characteristics, i.e., critical dimension (CD) uniformity, focal-depth or focus accuracy, overlay accuracy, and throughput. In order to improve the critical dimension uniformity, a wafer or a mask stage needs to improve its horizontal positioning precision. In order to improve the focal-depth accuracy, the wafer or mask stage needs to improve its vertical positioning precision. In order to improve the overlay accuracy, the wafer or mask stage needs to have an improved internal model which can enhance its dynamic positioning characteristics. In addition, considering the photolithographic apparatus is generally required to increase its throughput, the stage should also possess the capabilities of high-speed movement and rapid start and stop. The high-speed, high-acceleration and high-precision-positioning requirements of the photolithographic apparatus are conflicting, and to increase its scanning speed requires more powerful motors. In order to address this contradiction, the existing wafer stage technologies adopt both coarse and fine motion systems, so as to decouple the high-speed requirement from the high-precision requirement. Usually, the coarse-motion system is mostly realized by a linear motor which can enable large-stoke and high-speed movement, while the fine-motion system is generally mounted on top of the coarse-motion system, for dynamically compensating positional deviations. The fine-motion system is generally capable of a precision on the nanometer order and can move in multiple degrees of freedom to implement required photolithographic exposure and alignment tasks. However, since such structure currently adopts an air-bearing-based drive technology, it is not able to achieve the reconciliation of multiple-degree-of-freedom movement and actuator integration. This results in a large moving mass of the system and hence a large driving force, as well as significant residual vibration caused by a proportionally large reaction force to the driving force which is detrimental to the system's dynamic performance. In addition, considering that a high throughput requires the system to have a high acceleration which will cause a large additional tilting moment, the air suspension means for the wafer stage is generally designed to be subject to high static stiffness constraints, thus imposing extremely high requirements on guide surface planarity, preload deformation and air suspension parameters. Furthermore, after equipped with electrical, gas, liquid and vacuum passages, the motor case and other supporting components, the wafer stage system will become complex, huge, lack of reliability and difficult for maintenance.
A linear motor can drive a load to make a translational movement without needing the aid of any mechanical conversion means. Therefore, the linear motor is free of errors caused by deformation, backlash, or other factors of the conversion means, and has a relatively small inertia. A Halbach array is a novel arrangement of permanent magnets, in which the magnets having different directions of magnetization are arranged in a certain order such that the magnetic field on one side of the array is significantly augmented while the field on the other side is significantly canceled out. Therefore, it is easy for the Halbach array to form a desirable sinusoidally distributed spatial magnetic field and the array has thus found extensive application in linear motors due to its beneficial characteristics.
Kim W. J. of the Massachusetts Institute of Technology (MIT) describes in his PhD thesis, entitled “Precision Planar Magnetic Levitation”, published in 1997, a high-precision positioning stage system based on four Halbach array linear motors which cooperate to enable six-degree-of-freedom motions needed in photolithography. The system operates with a positioning noise of 5 nm and acceleration capabilities in excess of 1 g (9.8 m/s2). In addition, Williams of MIT also describes in his PhD thesis, entitled “Precision six-degree-of-freedom magnetically levitated photolithography stage”, a high-precision positioning stage having a hybrid drive system composed of Halbach array linear motors and electromagnets.
U.S. Pat. Pub. No. 20040246458A1 published on Dec. 9, 2014 discloses a linear motor for a wafer or mask stage of a photolithographic apparatus. The linear motor has a high driving force, high efficiency and low normal force, and includes a first magnet plate, a second magnet plate arranged in parallel and opposite to the first magnet plate, and an armature, which includes three open coil units, interposed between the first and second magnet plates. The first and second magnet plates and the coil units are relatively movable. The use of two opposed magnet plates and an open coil unit which does not include an iron core enables volume reduction of the motor without additional heat generation for an increase in the force output. Since this design leads to additional effective magnet material and hence higher magnetic forces, efficiency of the motor can be increased with the number of used bearings and the moving mass being both reduced, thus rendering the motor particularly useful in vacuum environments.
However, in the Halbach arrays of the two linear motors described above, an inconsistency of in-plane magnetic flux density distribution profile and geometric profile of the magnetic arrays will limit their vertical and horizontal magnetic fluxes. These limitations will lead to limited vertical and horizontal magnetic flux density as well as limited vertical and horizontal driving forces and hence limited driving forces in the six degrees of freedom.